Ink jet printers create a printed image on a surface by ejecting ink through orifices in a print head face plate onto the surface. To provide fine image resolution, the ejected ink droplets are very small, as are the orifices. Thus, an orifice may be partially or completely blocked by a small particle, thereby impairing printing. Paper fibers are commonly released into the printer environment during printing, and are the cause of some orifice obstructions. Paper fibers and other debris on a surface to be printed may be transferred to the print head face plate during printing, because the print head is positioned close to the surface to minimize droplet mispositioning due to off axis shooting that would cause an imprecise image.
Some ink jet printers employ a wax-like phase change ink that is melted for ejection onto a liquid intermediate transfer layer on a support surface of a rotatable transfer drum to form the image. When a complete image is formed, it is then transferred to a media sheet. Such printers preferably use a silicone liquid film as liquid intermediate transfer layer on the drum to facilitate release of the image to the sheet. However, the liquid film may collect debris and transfer it to the print head face plate. In addition, such printers require ventilation to remove heat generated by the ink melting apparatus and other heated elements. This keeps the support surface of the rotatable transfer drum at a selected temperature moderately above ambient, and cools internal electronic components. The resulting internal airflow may entrain contaminant particles and deposit them onto the face plate, onto media sheets, or onto transfer surfaces that subsequently carry them to the face plate.
While infrequent, an obstructed orifice nonetheless causes unacceptable printing, but may usually be cured by purging all orifices. This is generally effective, but consumes ink resources, takes time, and gives a reduced perception of quality by the user. In much rarer instances where the obstructed orifices are not cured by a purge, professional service may be required. Printers having permanent print heads benefit most from the avoidance of obstructions that might be more readily cured in printers with disposable inexpensive print heads that are customer replaceable.
The apparatus disclosed herein avoids or reduces the above limitations by providing a computer printer having a frame contained within a housing, a media transport and printing mechanism attached to the frame within the housing, and at least one debris shield above the printing mechanism to protect it from falling debris. The at least one debris shield may be positioned below an air flow path generated by printer ventilation, and the printer may employ a transfer surface on which an image is generated prior to transfer to a media sheet, and above which the debris shield is positioned. Where a plurality of debris shields are employed, at least one is transparent to facilitate locating and removing media sheet jams.